The present invention relates to a hollow cam shaft for use in an internal combustion engine.
Recently, an internal combustion engine has been required to be lightweight for saving energy while required to be high in its speed as well as in output. A cam shaft also has been necessarily required to be lightweight and to have superior in wear resistance.
Therefore, a variety of light weight and high wear resistance cam have been proposed each characterized by a steel pipe shaft assembled with a sintered cam piece or a sintered journal piece.
In conventional cam shafts, such as is shown in FIG. 1, a cam shaft is provided in which a journal portion 12 has an outer diameter smaller than that of a cam portion 14. In this case, the difference in diameter between a pipe 10 and the journal portion 12 is small. Such a cam shaft is disadvantageous in that the journal piece is so thin that it is extremely difficult to form by powder compacting and it is also difficult to assemble the thin journal piece with the pipe. The journal piece is easily broken because the journal piece is made of a powder compact. Therefore, in order to increase the thickness of the journal piece, it is necessary to make the diameter of the pipe small. If the pipe is reduced in diameter, the strength of the pipe becomes weaker correspondingly. In order to maintain the strength of the pipe, it is necessary to decrease the inner diameter of the pipe to obtain a thick walled pipe. Thus, the advantage of reduction in weight due to the use of a hollow pipe is diminished. Further, the cam piece becomes thicker, which leads to increased overall weight as well as increased cost of the cam shaft.